In a mobile communications system, wireless channel characteristics such as co-channel interference signals between or within cells, multipath fading, and the Doppler effect degrade the performance and capacity of the system. A smart antenna technology is used to increase the overall system capacity to counteract such causes of performance degradation. A smart antenna technology is the combination of array antennas formed of multiple antenna sensors spaced a part from each other at predetermined distances and a baseband signal processing technique, and provides more freedom of design by adding spatial processing capability to the system, thereby enhancing the overall system performance.
One of main functions of the smart antenna technology is beamforming. A beamforming technology is for creating the radiation pattern for a beam to direct in a particular direction, and includes a switched beamforming technique and an adaptive beamforming technique.
The switched beamforming technique uses a fixed set of weighting vectors for a number of directions, and the adaptive beamforming technique updates weighting vectors in accordance with the desired directions in order to maximize the ratio of the wanted signal to an interference signal.
Furthermore, the beamforming technology generally uses Butler matrix or Rotman lens. The former has the advantage of easy implementation when forming a small number of beams, whereas it has the disadvantages of complexity of the structure and the high manufacturing cost since it needs more hybrid couplers, fixed phase shifters, and crossovers as the number of beams to be formed increases.
On the other hand, the latter has the advantage of no limitation in the number of beams to be formed, a low manufacturing cost and operation in a wider band, whereas it has the disadvantage of poor precision in design of a focal arc, an array curve, constrained lines, and so on.